Many current blood systems process a number of fluids and have complex fluid path arrangements and they often use one or more cassettes to organize the lines and fluid paths. The cassettes organize the fluid lines, allow for a smaller overall system, and greatly simplify device loading and setup. An example of blood processing is apheresis. Apheresis is the process of removing whole blood from a patient or donor, separating the blood into its various components, removing one or more of the components, and returning the remaining components to the patient.
During blood processing, the pressure within the fluid lines is critical to patient safety and system performance and efficiency. That that end, blood processing systems typically monitor the pressure within some or all of the fluid lines (especially the draw and return lines). In an apheresis device, for example, the donor pressure may be monitored to ensure that it does not go above or below a threshold during withdrawal of whole blood and return of blood components. A few solutions to pressure monitoring have been implemented but each has significant drawbacks.
Some prior art systems have a monitoring line connected to the fluid line. The monitoring line may contain a 0.2 micron filter and may be manually connected to a pressure transducer by way of a tapered luer fitting. As the pressure within the fluid line increases, the fluid compresses a column of air trapped in the monitoring line between the flowing fluid and the transducer. The pressure transducer then detects the change in pressure. The system can detect a drop in pressure in a similar manner. Although this approach has been proven effective, it has several drawbacks. First, such designs may not easily be incorporated into a cassette. Additionally, these designs require the operator to connect to each transducer manually, making it prone to bad connections. If the connection is not air-tight, fluid may force the air out of the column and wet the transducer protector. If this happens, the sensor/transducer will no longer function.
Other prior art systems have taken a different approach. These systems have a flexible membrane (silicone, for example) within the fluid path. The membrane is in contact with the fluid on one side and a transducer on the other. Increases in pressure within the fluid line create a pressure on the transducer through the flexible membrane. However, these systems will not detect a negative pressure without the presence of a metallic disk attached to the transducer side of the membrane. The systems may then use a magnet, incorporated into the membrane, to create a coupling to the membrane even during negative pressure. This approach also has been proven effective but has a significant disadvantage in cost and complexity. Both the membrane and metallic disk add cost to the set and add complexity to the manufacturing and set-up process.